1. Field of the Invention
The present invention relates to a device for shielding electromagnetic radiation.
2. Discussion of the Background
The problem of designing screens for electromagnetic radiation has a long history. Since the development of radio engineering and electronics, the question of suppression of electromagnetic communication between separate circuits and units has been dealt with. Of equal concern is the reduction of the influence of electromagnetic radiation on surrounding wildlife and humans.
Conventional screens include metallic structures (metal sheets, a film, a grid etc.) and radio absorbing materials, as disclosed by O. S. Ostrovsky, et al.: “Filters and absorbers of electromagnetic waves,” ΦIΠ ΦΠ PSE (Ukraine), 2003, Vol. 1, No 2, pp. 161-173. Significant progress in the area of radio absorbing materials has been achieved for last 30 years in connection with works on a direction which usually designated as “Stealth”. Screens including a composite magnetic-dielectric in a microwave range have been developed thin enough (thickness about units of millimeters), broadband (more an octave) radio absorbing coverings which can be applied to electromagnetic shielding successfully. However, it was known that in order for screens of both types to effectively shield electromagnetic radiation, their cross-section sizes should be equal to many wave-lengths. Over the last decade, and in particular in connection with development of mobile communication, the need for screens that are small in comparison to length of a wave for shielding radiation has grown. Simple reduction of the cross-sectional sizes of metallic and radio-absorbing screens does not give an essential positive effect because electromagnetic waves may easily flow around such screens. For example, the miniscreen “Wave Buster” (South Korea) made in the form of a 15 mm diameter tablet that is 4 mm thick on the basis of layered radio-absorbing material decreases the radiation of the aerial of all on 10-20%, i.e. less than on 1 dB. And as disclosed by Jiunn-Nan Hwang, et al.: “Reduction of the Peak SAR in the Human Head with Metamaterials,” IEEE Trans. on Antennas and Propag., 54, December 2006, the plate of a radio-absorber (ferrite) of the greater area (45×45×6 mm) decreases the radiation on 6 dB for frequency of 0.9 GHz.
Radical change of the situation with the shielding of small aerials should be connected to publications of D. Sievenpiper and others (see U.S. Pat. No. 6,483,480, D. Sievenpiper, et al.; Sievenpiper D. et al.: “High-Impedance Electromagnetic Surfaces with a Forbidden Frequency Band,” IEEE Trans. on Microwave Theory and Tech., vol. 47, November 1999, pp. 2059-2074; Jimenez Broas, et al.: “High-Impedance Ground Plane Applied to Cellephone Handset Geometry,” IEEE Trans. on Microwave Theory and Tech., v. 49, July 2001, pp. 1262-1265) in which surfaces with a high surface impedance are considered. Such surfaces differently name artificial magnetic conductors (AMC) as at falling an electromagnetic wave on this surface the tangential magnetic field has a node instead of an antinode, as on a surface of an electric conductor. In particular, it is shown that behind such surfaces, even with a limited cross-section size, the area of a deep electromagnetic shadow is formed, i.e. the surface is the screen of electromagnetic radiation. Also, it has been shown that application of a resonance high impedance surface as a ground plane of aerials increases its directivity. As disclosed by Jimenez Broas R. F. et al., for the model of the mobile phone antenna located on an AMC with sizes 25×50 mm, the “forward-back” ratio for a 2.5 GHz signal was about 10 dB.
Attempts to apply a metamaterial on the basis of double open rings to shielding have shown that the structure from 10 layers in volume 34×34×15 mm has an attenuation of 3 dB for 1.8 GHz, as disclosed by Jiunn-Nan Hwang, et al.
U.S. Pat. No. 6,483,480, which is incorporated herein by reference, discloses a tunable impedance surface. The one includes two two-periodic lattices from the metal squares parallel to a metallic ground on distance small in comparison with the length of a wave. The distance between lattices also is small in comparison with length of a wave. Each element of the first lattice nearest to a metal substrate is connected to a metal ground. This lattice is motionless. The second lattice, which is mobile, can be shifted to the first lattice so that the distance between lattices remains constant. The impedance in a plane of a mobile lattice depends on a frequency of electromagnetic radiation and geometry of lattices. The surface impedance is maximal near to resonant frequency of the given system. The resonance of system is the resonance of a large number of the connected elementary resonators. Each elementary resonator is formed by capacity between two elements of the mobile and motionless lattices and also inductance of the circuit including a site of a metal substrate and 2 next conductors connecting elements of the motionless lattice with a metal substrate.
The tunable impedance surface disclosed in U.S. Pat. No. 6,483,480 may have a non-uniform surface impedance due to the limited size. Elementary resonators at edges of an impedance surface are connected to a smaller number of the next resonators than elementary resonators in the center of an impedance surface. Thus, resonant frequencies of elementary resonators and corresponding local impedances depend on location of these resonators. An undesirable result of the non-uniform impedance is the “washing out” of the effect of shielding, (i.e. its reduction on working frequencies of the screen). Another problem with the tunable impedance surface disclosed in U.S. Pat. No. 6,483,480 is the complexity of the design in which a large number of shorting connections of all elements of a motionless lattice are connected with a conducting substrate.